1_horror.py

def get_inventory_events(self,

    stock_items = self.stockitem_set.all()

    qs = InventoryEvent.objects.filter(
        Q(booking_stock_item__stock_item__in=stock_items) |
        Q(booking_cross_hire__stock_item__in=stock_items) |
        Q(purchase__stock_item__in=stock_items) |
        Q(cross_hire__stock_item__in=stock_items) |
        Q(repair__stock_item__in=stock_items) |
        Q(missing__stock_item__in=stock_items)
    )

    return qs

2_horror.sql

SELECT "inventory_inventoryevent"."id"
FROM "inventory_inventoryevent"
LEFT OUTER JOIN "office_bookingstockitem" ON ( "inventory_inventoryevent"."booking_stock_item_id" = "office_bookingstockitem"."id" )
LEFT OUTER JOIN "office_bookingcrosshire" ON ( "inventory_inventoryevent"."booking_cross_hire_id" = "office_bookingcrosshire"."id" )
LEFT OUTER JOIN "inventory_purchase" ON ( "inventory_inventoryevent"."purchase_id" = "inventory_purchase"."id" )
LEFT OUTER JOIN "inventory_crosshire" ON ( "inventory_inventoryevent"."cross_hire_id" = "inventory_crosshire"."id" )
LEFT OUTER JOIN "inventory_repair" ON ( "inventory_inventoryevent"."repair_id" = "inventory_repair"."id" )
LEFT OUTER JOIN "inventory_missing" ON ( "inventory_inventoryevent"."missing_id" = "inventory_missing"."id" )
WHERE ("office_bookingstockitem"."stock_item_id" IN (
    SELECT "inventory_stockitem"."id"
    FROM "inventory_stockitem"
    WHERE "inventory_stockitem"."item_model_id" = 81
) OR "office_bookingcrosshire"."stock_item_id" IN (
    SELECT "inventory_stockitem"."id" 
    FROM "inventory_stockitem" 
    WHERE "inventory_stockitem"."item_model_id" = 81
) OR "inventory_purchase"."stock_item_id" IN (
    SELECT "inventory_stockitem"."id"
    FROM "inventory_stockitem"
    WHERE "inventory_stockitem"."item_model_id" = 81
) OR "inventory_crosshire"."stock_item_id" IN (
    SELECT "inventory_stockitem"."id"
    FROM "inventory_stockitem"
    WHERE "inventory_stockitem"."item_model_id" = 81 
));

3_explain.txt

                                                                QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------
 Hash Left Join  (cost=203.42..449.19 rows=5308 width=4)
   Hash Cond: (inventory_inventoryevent.cross_hire_id = inventory_crosshire.id)
   Filter: ((hashed SubPlan 1) OR (hashed SubPlan 2) OR (hashed SubPlan 3) OR (hashed SubPlan 4))
   ->  Hash Left Join  (cost=148.53..373.04 rows=5662 width=20)
         Hash Cond: (inventory_inventoryevent.purchase_id = inventory_purchase.id)
         ->  Hash Left Join  (cost=81.75..270.32 rows=5662 width=20)
               Hash Cond: (inventory_inventoryevent.booking_cross_hire_id = office_bookingcrosshire.id)
               ->  Hash Left Join  (cost=57.58..224.91 rows=5662 width=20)
                     Hash Cond: (inventory_inventoryevent.booking_stock_item_id = office_bookingstockitem.id)
                     ->  Seq Scan on inventory_inventoryevent  (cost=0.00..105.62 rows=5662 width=28)
                     ->  Hash  (cost=36.70..36.70 rows=1670 width=8)
                           ->  Seq Scan on office_bookingstockitem  (cost=0.00..36.70 rows=1670 width=8)
               ->  Hash  (cost=16.30..16.30 rows=630 width=8)
                     ->  Seq Scan on office_bookingcrosshire  (cost=0.00..16.30 rows=630 width=8)
         ->  Hash  (cost=40.79..40.79 rows=2079 width=8)
               ->  Seq Scan on inventory_purchase  (cost=0.00..40.79 rows=2079 width=8)
   ->  Hash  (cost=15.10..15.10 rows=510 width=8)
         ->  Seq Scan on inventory_crosshire  (cost=0.00..15.10 rows=510 width=8)
   SubPlan 1
     ->  Index Scan using inventory_stockitem_item_model_id on inventory_stockitem  (cost=0.28..8.35 rows=4 width=4)
           Index Cond: (item_model_id = 81)
   SubPlan 2
     ->  Index Scan using inventory_stockitem_item_model_id on inventory_stockitem inventory_stockitem_1  (cost=0.28..8.35 rows=4 width=4)
           Index Cond: (item_model_id = 81)
   SubPlan 3
     ->  Index Scan using inventory_stockitem_item_model_id on inventory_stockitem inventory_stockitem_2  (cost=0.28..8.35 rows=4 width=4)
           Index Cond: (item_model_id = 81)
   SubPlan 4
     ->  Index Scan using inventory_stockitem_item_model_id on inventory_stockitem inventory_stockitem_3  (cost=0.28..8.35 rows=4 width=4)
           Index Cond: (item_model_id = 81)